Mirrors the native PollingStatus streamed by iOS PollingCollector.poll()
(AsyncStream<PollingStatus>) and Android PollingCollector.pollStatus()
(Flow<PollingStatus>). continue is an intermediate tick; complete,
timedOut, expired, and error are terminal — no further ticks are
emitted for the same subscriptionId after one of these fires.
pollStatus does not auto-advance the flow on a terminal status — the
consumer must call next() explicitly to progress past it.
Discriminated union of streamed polling status events emitted by DaVinciClient.pollStatus.